The nearly planar and almost circular orbits of the planets in our Solar System argue strongly for planetary formation within flattened circumstellar disks. There is convincing observational evidence that stars form by gravitationally-induced compression of relatively dense regions within molecular clouds (Lada, Strom, & Myers 1993; Andre, Ward-Thompson, & Barsony 2000). Observations by Goodman et al. (1993) indicate that typical star-forming dense cores inside dark molecular clouds have specific angular momentum > 1021 cm2 . When these clouds undergo gravitational collapse, this angular momentum leads to the formation of pressure-supported protostars surrounded by rotationally supported disks. Such disks are analogous to the primordial solar nebula that was initially conceived by Kant and Laplace to explain the observed properties of our Solar System (e.g., Cassen et al. 1985). Observational evidence for the presence of disks of Solar System dimensions around pre-main sequence stars has increased substantially in recent years (McCaughrean, Stepelfeldt, & Close 2000). The existence of disks on scales of a few tens of astronomical units is inferred from the power-law spectral energy distribution in the infrared over more than two orders of magnitude in wavelength (Chiang & Goldreich 1997). Observations of infrared excesses in the spectra of young stars suggest that the lifetimes of protoplanetary disks span the range of 106-107 years (Strom, Edwards, & Skrutskie 1993; Alencar & Batalha 2002).

Our understanding of planet formation, such as it is, comes from a diverse set of observations, laboratory studies and theoretical models. Detailed observations are now available for the planets and many smaller bodies (moons, asteroids and comets) within our Solar System. Studies of meteorite composition, minerals and physical structure have been used to deduce conditions within the protoplanetary disk (Hewins, Jones, & Scott 1996). Data on extrasolar planets are less detailed and more biased, yet still very important. Moreover, our Solar System includes the intrinsic bias of containing a habitat where life can evolve to the point of asking questions about other worlds (Wetherill 1994). Observations of young stars and their surrounding disks provide clues to the planet formation now taking place within our galaxy. Laboratory experiments on the behavior of hydrogen and helium at high pressures have been combined with gravitational measures of the mass distribution within giant planets deduced from the orbits of natural satellites and the trajectories of passing spacecraft to constrain the internal structure and composition of the largest planets within our Solar System.

Theorists have attempted to assemble all of these pieces of information together into a coherent model of planetary growth. But note that planets and planetary systems are an extremely heterogeneous lot, the 'initial conditions' for star and planet formation vary greatly within our galaxy (MacLow & Klessen 2004), and at least some aspects of the process of planet formation are extremely sensitive to miniscule changes in initial conditions (Chambers et al. 2002).

The remainder of this chapter is organized as follows: Observations are summarized in Section 2. Accretion models for terrestrial planets around single stars are reviewed in Section 3, and simulations of terrestrial planet formation around binary stars are summarized in Section 4. Giant planet formation models are discussed in Section 5, with conclusions presented in Section 6.

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